1. Field of the Invention
The present invention relates to signal lines, and more specifically to signal lines including ground conductors and signal lines.
2. Description of the Related Art
Examples of existing inventions regarding signal lines include a printed wiring substrate disclosed in Japanese Unexamined Patent Application Publication No. 2009-54876. FIG. 5 is a cross-sectional structure diagram of a printed wiring substrate 500 disclosed in Japanese Unexamined Patent Application Publication No. 2009-54876. In FIG. 5, the z-axis, y-axis, and x-axis are respectively defined to be the up-down direction, the left-right direction, and a direction perpendicular to the sheet.
Referring to FIG. 5, the printed wiring substrate 500 includes an insulating layer 502, a signal line 504, and electrode planes 506 and 508. The signal line 504 extends in the x-axis direction within the insulating layer 502. The electrode plane 506 is arranged on the positive z-axis direction side of the signal line 504. The electrode plane 508 is arranged on the negative z-axis direction side of the signal line 504. Further, the electrode plane 508 has a line-shaped opening portion 510 arranged therein so as to be overlapped by the signal line 504. A high-frequency signal is transmitted through the signal line 504. The ground potential is applied to the electrode planes 506 and 508. In other words, the signal line 504 and the electrode planes 506 and 508 form a strip line structure.
In the printed wiring substrate 500 configured as described above, the printed wiring substrate 500 can be easily bent. In more detail, the line-shaped opening portion 510 is provided in the electrode plane 508. Hence, the electrode plane 508 can elastically stretch and shrink more easily than the electrode plane 506, which does not have the line-shaped opening portion 510 provided therein. As a result, the printed wiring substrate 500 can be easily bent.
However, the printed wiring substrate 500 has a problem in that high-frequency signal loss is generated. In more detail, when a high-frequency signal is transmitted through the signal line 504, a magnetic field passing through the electrode planes 506 and 508 is generated around the signal line 504. Since a current varies periodically in a high-frequency signal, the generated magnetic field also varies periodically. When the magnetic field varies periodically in this manner, an eddy current is generated in the electrode planes 506 and 508 by electromagnetic induction to counteract the variations of the magnetic field. As a result, eddy current loss is generated in a signal transmitted through the signal line 504.